Add Cow-based internal tokenizer functions for zero-copy preparation

Add internal _borrowed() functions that return Cow<'a, str> to prepare for
zero-copy tokenization. When the source string needs no transformation
(no escaping), return Cow::Borrowed. When transformation is required,
return Cow::Owned.

The Token enum still uses String, so borrowed values are converted via
to_owned() for now. This maintains API compatibility while preparing the
codebase for a future refactor where Token can hold borrowed strings.

Optimized: comments, quoted strings, dollar-quoted strings, quoted identifiers.

Add safe_slice() helper function for bounds-checked string slicing.

Author: eyalleshem